Pavement markings, such as those on the centerline and edgeline of a roadway, are important in order to provide visual guidance for motor vehicle drivers. Pavement marking materials are used as traffic control markings for a variety of uses, such as short distance lane striping, stop bars, and pedestrian pavement markings at intersections. A common form of pavement markings is adhesive-backed tape that is applied to the roadway surface in desired location and length; the top surface of the tape having selected color and typically retroreflective characteristics.
Currently, many flat pavement markings rely on an exposed-lens optical system comprising transparent microspheres partially embedded in a binder layer containing pigment particles, e.g., titanium dioxide (TiO.sub.2) or lead chromate (PbCrO.sub.4), as reflectors. In use, light from the headlamp of a vehicle enters the microsphere and is refracted to fall on the reflective pigment particles. Some portion of the light is returned generally along the original entrance path in the direction of the vehicle so as to be visible to the driver. The amount of refraction and the amount of light gathering of these microspheres is dependent in part upon maintaining a low index of refraction at the air interface on the exposed portion of the microsphere. During rainy periods, the microspheres become wetted with water which reduces the their light refracting ability and results in much reduced retroreflective performance.
One solution to this problem is raised pavement markings in which retroreflective elements are presented in somewhat vertical configurations. U.S. Pat. Nos. 4,388,359 (Ethen et al.), 4,988,555 (Hedblom), and 4,988,541 (Hedblom) disclose pavement markings with protrusions carrying exposed-lens retroreflective elements on the sides thereof.
It is also known to use enclosed-lens retroreflective structures on pavement markings. These structures are typically used as spot guides which are augmented with continuous paint or tape markings. See, e.g., U.S. Pat. Nos. 5,277,513 (Flanagan et al.) and 5,340,231 (Steere et al.). Enclosed-lens retroreflective sheetings with flat cover films (also sometimes referred to as cover sheets, top sheets, top films, etc.) have been constructed as a means for improving wet retroreflectivity. See, e.g., U.S. Pat. No. 4,025,159 (McGrath) which discloses encapsulated-lens retroreflective articles and U.S. Pat. Nos. 4,505,967 (Bailey) and 4,664,966 (Bailey et al.) which disclose embedded-lens retroreflective articles.
U.S. Pat. No. 4,145,112 (Crone) discloses an article comprising an underlying base retroreflective layer and a light directing layer comprised of a longitudinally-extending series of short transparent projections which each have front and back (defined relative to the expected origin of light to be retroreflected) upwardly extending edge surfaces. The front edge surfaces are disposed across (i.e., relatively perpendicular to) the expected path of high-incidence angle light, whereby they transmit rather than reflect away a high percentage of incident light from approaching motor vehicles. The back edge surface is disposed to both reflect light transmitted through the front edge surface to a path within a predetermined angular range for retroreflection by the retroreflective elements, and to reflect light retroreflected by the retroreflective elements back through the front edge surface toward its source. A precise configurational relationship of the front and back upwardly extending edge surfaces of each projection must be established and maintained in order to retain adequate retroreflectivity. In addition, the longitudinally extending projections tend to make such sheeting less flexible. U.S. Pat. No. 4,236,788 (Wyckoff) discloses a related type of pavement marker strip wherein the two sides of the transverse prisms are adjusted to provide for downward internal reflection into the base sheet from one side and refraction to the space between successive prisms into the base sheet from the other side. As with the article disclosed in U.S. Pat. No. 4,145,112, maintenance of a precise configurational relationship between the two upward faces of the prisms is critical.
U.S. Pat. No. 3,920,346 (Wyckoff) discloses a saw-tooth-like marker strip comprising protrusions with curved edges and having upwardly disposed retroreflective members embedded therein. The curved edges of the raised protrusions are said to reduce loss of incident light so that the marker is bright over wide angles of incident light upon the marker strip. In addition, incorporation of upwardly disposed retroreflective members in the protrusions results in a narrower entrance or incidence angle of light from approaching motor vehicles, permitting more effective retroreflection by the article.
U.S. Pat. No. 4,072,403 (Eigenmann) discloses a retroreflective assembly that is particularly useful for situations in which retroreflection is required in rainy conditions. The assembly disclosed therein comprises a transparent globule with a monolayer of transparent microspheres on certain portions of the globule and a reflective layer disposed behind the microspheres. The retroreflective assemblies, sometimes referred to as "globule/microsphere retroreflective assemblies", are positioned on the top surface of a pavement marking where they provide improved retroreflection of light at high incidence angles. U.S. Pat. No. 5,268,789 (Bradshaw) teaches an improved globule/microsphere retroreflective assembly and an improved method for making such an assembly.
EP Patent No. 385746 B1 (Kobayashi et al.) discloses a pavement marking comprising a layer of large glass microspheres embedded on top of retroreflective embedded-lens type base sheeting. The retroreflective pavement marking is said to be particularly useful in rainy conditions because the larger glass microspheres are partially exposed in air. However, the disclosed pavement marking is limited to using microspheres as a light gathering source. Furthermore, the pavement marker is only taught to increase retroreflectivity of its base sheeting at entrance angles of between 60.degree. and 80.degree.. It is known in the art that high-entrance angles, greater than approximately 85.degree., is more common for pavement marking applications.
Currently available pavement markings provide effective retroreflective response for only a narrower range of entrance angles than is sometimes desired. In addition, currently available pavement markings are not as effective retroreflectors as is desired for some applications. For example, current commercial flat pavement markings, relying on microspheres partially embedded in layers containing pigment particles, are most easily seen at distances of approximately 80 meters and less. At distances greater than this, retroreflective brightness declines due to the relatively larger entrance angles of the incident light and limited retroreflective efficiency. In addition to generally low retroreflectivity at high incidence angles, flat pavement markings are particularly difficult to see under rainy conditions. Raised pavement markings have better wet reflectivity because the rain will run off the raised portions. However, snow removal is frequently a problem on roads bearing raised pavement markings, as the snowplows have a tendency to catch on the raised protrusions and dislodge the markings from the road surface.
The need exists for low profile retroreflective articles that exhibit high retroreflective brightness in a continuous line even at high incidence angles and which retain effective retroreflective brightness at high incidence angles even when wet. As used herein, "low profile" refers to an article low enough to sustain impacts from a snow plow after one winter season with minimal damage to the article. In addition, there exists a need for retroreflective articles exhibiting effective retroreflective response over a wide range of entrance angles for application to vertical surfaces such as guard rails, Jersey barriers, etc.